The invention relates to a dual-circuit master cylinder, in particular for hydraulic braking systems of automotive vehicles which includes two bores extending beside each other in a casing and which with the ends of a first and a second power piston form a first and a second power chamber linked to wheel brakes, which power pistons are actuatable by a power output member via a yoke interconnecting the two power pistons.
In a known dual-circuit master cylinder of the kind referred to above (German printed and published patent application No. 1655266), a power output member actuatable by a brake pedal acts on the yoke which in turn actuates the two power pistons. The yoke operates on the principle of a scale beam and by a tipping action balances different travels covered by the power pistons. This tipping action causes, however, a nonuniform distribution of power to the two power pistons so that different pressures occur in the power spaces associated with the power pistons. In the event of a failure of one of the two pressure medium circuits, a friction-type locking action occurs between the power output member, the yoke and the power piston. This locking action leads to considerable loss of efficiency, as well as an increase in the stroke of the yoke.
In this known dual-circuit master cylinder, the travel covered by the power output member equals that covered by the power pistons. The loss of travel due to the serial arrangement of the power pistons has its effect particularly in the operating range extending from the start of the actuation to the pressure rise in the power chambers associated with the individual wheel brakes. This increased travel loss before application of the brakes causes a reduction of the effective operating range at the brake pedal of a vehicle.
Another dual-circuit master cylinder (German patent specification No. 2460529) provides for a larger travel at the power piston for a given travel of the brake pedal. However, a reduction of the travel to be covered by the brake pedal is partially offset by the fact that the travels of the power pistons arranged one behind the other add up at the brake pedal. As a consequence, this dual-circuit master cylinder has a relatively large overall length.